Referring to FIGS. 1 and 2, a conventional spring actuated fastener driving tool 1 as disclosed in Taiwanese Patent Publication No. 545350 includes a housing 11, a nail striking unit 12 and a handle unit 13. The housing 11 has a central shaft 111. The nail striking unit 12 has a striking member 121 mounted in a front portion of the housing 11 for reciprocal movement in an upper-and-lower direction, and a leaf-spring 122 disposed in the housing 11 and connected at a front end to the striking member 121. The handle unit 13 has a handle 131 pivotably mounted to the central shaft 111, and a compressible spring 132 disposed in the housing 11 to be compressed to produce a biasing force to the handle 131. The handle 131 is engageable with the striking member 121 at a front end thereof.
When the handle 131 is pivoted about the central shaft 111 by a manual press force applied on a rear end thereof to move the striking member 121 upwardly, the leaf-spring 122 is resiliently deflected and loaded. Then, a successive pivot movement of the handle 131 results in disengagement thereof from the striking member 121, and at this moment the leaf spring 122 is thus freed to resiliently return the striking member 121 downwardly to eject a fastener.
However, during pivot movement of the handle 131, the leaf-spring 122 must be resiliently deflected by a relatively long distance to accumulate a sufficient biasing energy. Thus, a large operating effort is required to move the striking member 121 for the long distance to bend the leaf-spring 122 to a sufficient degree.